Tsarnaev made the remark when an investigator working for his lawyers accompanied Tsarnaev's sister to a prison visit, a meeting that was monitored by an FBI agent, prosecutors said. The defense investigator started to explain to Tsarnaev's sister the rationale behind special restrictions placed on Tsarnaev in prison, prosecutors said.

They say Tsarnaev, "despite the presence of an FBI agent and an employee of the Federal Public Defender, was unable to temper his remarks and made a statement to his detriment which was overheard by the agent."

Boston Recovers

The government described the conversation in a memo outlining their opposition to a request from Tsarnaev's lawyers to lift the prison restrictions, known as "special administrative measures."

Tsarnaev, 20, has pleaded not guilty in a terror attack at last year's marathon. Two pressure cooker bombs were placed near the marathon finish line, killing three people and injuring more than 260.

Prosecutors have said they will seek the death penalty against Tsarnaev for crimes that include using a weapon of mass destruction.

Prosecutors argued that the FBI agent's presence was permitted by the special administrative measures, which prohibit providing information to people outside the prison. Tsarnaev's lawyers have argued that the measures limit Tsarnaev's interactions with people helping his defense team.

Tsarnaev's lawyers say the presence of the FBI agent during prison visits by Tsarnaev's two sisters "has thwarted the defense ability to develop important mitigation information."

They argue that courts have recognized the kind of information the defense wants to develop as admissible mitigation evidence, including evidence on "family dysfunction, mental illness and the impact of family chaos on the defendant as he grew up."

Prosecutors have argued that the restrictions are necessary in Tsarnaev's case because of his "commitment to jihad" and his "widespread notoriety."

"There was no expectation of privacy on the part of Tsarnaev, his visitors or the investigator," they argue.

Prosecutors have alleged that Tsarnaev and his brother, Tamerlan Tsarnaev, 26, built the bombs and placed them near the finish line of the April 15 marathon. Tamerlan Tsarnaev died following a shootout with police several days after the bombing.